This invention relates to a circuit arrangement for the direct electric heating of melts having tonic conductivity, particularly vitreous materials or glass melts.
Electric melting of some e.g. lead oxide glasses is limited by heavy corrosion of molybdenum electrodes which leads to the contamination of the glass melt. On the other hand, the electric melting of this type of glass might be very desirable since it reduces the volatilization of substances harmful to human health (Pb O).
The amount of substances being capable of oxidizing molybdenum as e.g. lead oxide, refining and coloring agents--the so-called depolarizers--defines the corrodibility of the glass melt. Molybdenum is oxidized on account of the reduction of the depolarizer. The rate of corrosion is determined by the intensity of anodic partial current.
Another important corrosion factor is the loading of the electrode by alternating electric current. It has been proven that the corrosion in a glass melt is influenced not only by the power loading but also by the surface current density. The amount of corroded molybdenum and precipitated lead are approximately equivalent within broad limits of the alternating current loading.
In accordance with theory concerning the corrosion of metals by alternating current, the reason for accelerated corrosion is the increase of the mean value of the anodic partial, i.e. corrosive, current. The mean value of the cathodic partial current, being a measure of the rate of depolarization reactions, tends to be equally on the increase. In accordance with the above-mentioned theory, the corrosive effect increases with growing alternating current density, but decreases with increasing frequency. The reason for the decreasing effect with higher frequencies is the growing part of the reactive-capacity-current which does not participate in the electrode reactions.
The theoretical assumptions as to the conditions of electric melting of glass need not have been always fulfilled. Due to heavy current loadings of electrodes, some other different mechanism retarding corrosion may take part in the process. This finding is confirmed by the change of the shape of the polarizing curve.
These experimental, as well as practical, results described in current literature, have proved that electrode corrosion increases with increased alternating current density and decreases with increasing frequency. The frequencies used are considered to be 50 or more Hz.
The study of the dependence of molybdenum electrode corrosion in a glass melt upon frequency has shown that in a definite region of low frequencies--namely under 50 Hz--the corrosion as well as the amount of decomposed depolarizer may decrease with decreasing frequency. The curve (not shown here) which illustrates the dependence of the corrosion rate on frequency presents a minimum of corrosion at some optimal frequency.
The rate of corrosion at this optimal frequency may be suppressed under the the corrosion at the currentless stage. Below this optimal frequency the rate of corrosion tends to be on the increase again.
The dependence on frequency of the amount of decomposed depolarizers and of the corrosion of molybdenum in different glass melts has a complicated course with a maximum and minimum. The location of these two extremes depends on the current density and the temperature of the electrode surface.
The results may be generalized so that the rate of the corrosion process is in a broad range of conditions a function of the ratio between current density and frequency of the alternating current. This ratio is proportional to the quantity of the electric charge transferred in a half period.
The fact that there exists an optimal low frequency region in which the rate of corrosion decreases may be employed for the protection of electrodes against corrosion. It is an advantage of this method that there is no need for any auxiliary electrodes when an anodic or cathodic protection is being applied. Likewise it is unnecessary to make changes in the construction of the melting furnace.
Static power frequency changers (cycloconverters) generating an electrical output wave form having a frequency which differs from the input frequency are known. Such cycloconverters, usually provided with thyristors, are generally used for the control of the speed of asynchronous motors.
Their application for powering vitreous materials furnaces or glass melting furnaces is not known. The circuit arrangements of known cycloconverters are not suitable for this purpose.
It is an object of this invention to provide a circuit arrangements capable to supply vitreous materials melting furnaces by an alternating electric current with a frequency lower then 60 or 50 Hz. The circuit arrangements of the invention contain devices enabling the changing of a 3-phase input into a single phase output power and simultaneously the changing of the mains frequency into an optimal one with respect to the minimal corrosion of electrodes to adjust and control the optimal low frequency of the electric current feeding of furance, whereby to measure and/or control the optimal value of the direct current component of the feeding current, and to measure and/or control the power input of the furnace to a constant level or to maintain constant melting temperature.
In accordance with the invention, the preferred arrangements for for feeding powering a glass melting furnace employs a cycloconverter which consists of two 3-phase banks of thyristors interconnected on the DC side by reactors to make the shape of the output current wave smoother, although the wave shape does not play any important role in this case.
The input of the cycloconverter is connected to a threephase transformer having a mains frequency (50 or 60 Hz). The frequency of the single phase output is adjusted to a value of frequency at which the rate of corrosion of the electrodes in the furnace is suppressed to its minimum.
The ability to change the three-phase input into a single-phase output with a simultaneous change of frequency of the output alternating current is a feature of the cycloconverter which may be favorably exploited for powering (feeding) an electric glass melting furnace. The three-phase input of the cycloconverter guarantees the uniform loading of the three-phase mains, and the single-phase output, on the other hand, renders better distributed electric and power fields between the electrodes. The uniformly distributed fields favorably influence the flow of glass melt in the furnace. The change of the three-phase into a single-phase AC has been until now realized by a Scott transformer without, of course, having the possibility of changing the frequency.
The shape of the single-phase alternating current wave at the output of the cycloconverter plays a secondary role only. The corrosion of electrodes is also on the decrease when a nonharmonic AC of optimal frequency is being applied. On the other hand, it is important to keep the quantity of the electric charge transferred in each half-period (half wave) as equal as possible. The shapes of the positive and negative half-waves are not important, but their areas, i.e. the amounts of power delivered by each have to be as equal as possible. The value of the DC component is determined by the difference between the areas of the positive and negative half-waves in a period.
When the areas charges (amounts of power) of the positive and negative half-waves of a period (a cycle) of the low frequency AC are not identical, then the position of their "line of symmetry" differs from the "zero line".
It is, therefore, another object of the invention to measure and/or control the DC component of the power delivered to the furnace in order to stabilize the favorable anticorrosion effect of the optimal frequency.
The circulating currents, which may arise between positive and negative banks of the thyristors, due to a reactor connecting the positive and negative banks, are eliminated by introducing a time dwell between the positive and negative half-waves of the current. By prolonging the time of commutation, there is simultaneously created a proof against the short-circuiting of positive and negative banks of thyristors. The above mentioned means may have a partially negative influence on the shape of the output wave form. As mentioned before, the wave-form, however, is not critical with regard to the corrosion of the electrodes.